This invention relates to the art of hydraulic presses and, more particularly, to an improved arrangement for positively stopping travel of the displaceable component of the hydraulic drive unit in the direction of movement thereof toward the press bed.
It is of course well known to provide a hydraulic press with a hydraulic drive unit of the character comprising a ram fixed with respect to the press frame and a cylinder slidably displaceable relative to the ram and operatively connected with the press slide to impart reciprocation thereto relative to the press bed. Heretofore, in connection with such a hydraulic drive unit in an overhead drive arrangement for example, the ram is fixed to the crown portion of the press frame, and the drive cylinder component has an open upper end receiving the ram and defines a hydraulic fluid receiving variable volume chamber with the ram. The press slide is attached to the closed lower end of the cylinder and is displaced thereby toward the press bed by introducing hydraulic fluid between the ram and cylinder. Return movement of the cylinder and slide is achieved by hydraulic return cylinder units on the crown and connected to the cylinder. Heretofore, the open upper end of the cylinder component has been provided with a crosshead mechanically fastened to the cylinder such as by bolts extending through the crosshead and axially into the cylinder wall. Piston rods of the return cylinders are attached to the crosshead, and radially outer portions of the crosshead carry guide components engageable with gibbing on the press frame to provide support and guidance for the cylinder during reciprocation thereof.
Work with respect to a given workpiece is of course performed during movement of the cylinder and thus the slide toward the press bed. The extent of displacement of the cylinder toward the press bed is critical with respect to protecting the drive cylinder, return cylinders and guide components from damage, and with respect to achieving a desired cooperable interengagement between tooling on the slide and press bed. In this respect, overtravel of the cylinder and thus the slide in the direction toward the press bed, such as might be due to a hydraulic malfunction for example, can result in damage to the mechanical connection between the crosshead and drive cylinder and, thus, to the guide components and gibbing and to the hydraulic return cylinder components. Accordingly, it is necessary to control and limit displacement of the drive cylinder and thus the slide in the direction toward the press bed to avoid such potential damage. Clearance requirements within the die space area of the press prevent the use of any mechanical stop pin or similar type arrangement between the slide and bed of the press and, heretofore, control of the displacement of the drive cylinder and thus the slide was achieved through the use of a hydraulic flow control valve mechanically actuated upon movement of the cylinder crosshead and thus the slide to a desired position relative to the press bed. In this respect, the valve is actuated to dump hydraulic fluid under pressure from behind the drive cylinder, thus to prevent slide overtravel. Such a dump valve arrangement is effective when it functions properly, but such an arrangement is unreliable in this respect and, moreover, is undesirably expensive. With regard in particular to unreliability, the valve as mentioned above is mechanically actuated by the crosshead component on the cylinder, whereby it is extremely difficult to maintain accuracy of adjustment between the valve actuator and crosshead to achieve valve operation at the same time during each succeeding stroke of the drive cylinder and thus the press slide. Accordingly, considerable down time is required to maintain a satisfactory degree of accuracy with respect to such valve actuation and, when such accuracy is lost, either slide undertravel or overtravel can result. Overtravel is undesirable for the reason of the potential damage referred to hereinabove, and undertravel can result in inaccuracies with respect to the work performed on a workpiece in the press.